1. Field Of The Invention
This invention is related to an apparatus and method for reducing NO.sub.x emissions from premix burners, without altering critical heat distribution from the burners. This invention may be employed in high temperature furnaces, for example, for steam cracking hydrocarbons.
2. Description Of Background And Relevant Information
Various nitrogen oxides, i.e., NO.sub.x compounds, are formed in air at high temperatures; these include, but are not limited to, nitric oxide and nitrogen dioxide. Reduction of NO.sub.x emissions is a desired goal in order to decrease air pollution and meet government regulations.
Burners may use either liquid fuel or gas. Liquid fuel burners may mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and combustion air is mixed with the fuel at the point of combustion.
Gas fired burners are classified as either raw gas or premix, depending on the method used to combine the combustion air and fuel. These burners differ in configuration, and in the type of burner tip used.
Raw gas burners inject fuel directly into the combustion air stream, and the mixing of fuel and air occurs simultaneously with combustion.
Premix burners mix the fuel with some or all of the combustion air prior to combustion. Premixing is accomplished by using the energy of the fuel stream so that air flow is generally proportional to fuel flow. Therefore, frequent adjustment is not required and the achievement of desired flame characteristics is facilitated.
Floor-fired premix burners are used in many steam crackers and reformers, mainly for their ability to produce relatively uniform heat distribution in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Due to these properties, premix burners are widely used in various steam cracking furnace configurations.
MICHELSON et al., U.S. Pat. No. 4,629,413, discloses a low NO.sub.x premix burner and discusses the advantages of premix burners and methods to reduce NO.sub.x emissions; this patent is incorporated herein in its entirety, by reference thereto. The premix burner of MICHELSON et al. lowers NO.sub.x emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air.
BRAZIER et al., U.S. Pat. No. 4,708,638, discloses a fluid fuel burner, in which NO.sub.x emissions are reduced by lowering the flame temperature. A venturi in a combustion air supply passage, upstream of a swirler, induces the flow of flue gas into the combustion air supply passage from ducts opening into the furnace. A swirler is located at the free end of a fuel pipe and mixes the flue gas with the primary combustion air.
FERGUSON, U.S. Pat. No. 2,813,578, discloses a heavy liquid fuel burner, which mixes the fuel with steam prior to combustion. The aspirating effect of the fuel and steam draws hot furnace gases into a duct and into the burner block to aid in heating the burner block and the fuel and steam passing through a bore in the block. This arrangement is disclosed as being effective to prevent coke deposits on the burner block and also to prevent any dripping of the oil. Since the flame temperature is raised, this arrangement would not aid in reducing NO.sub.x emissions.
JANSSEN, U.S. Pat. No. 4,230,445, discloses a fluid fuel burner, which reduces NO.sub.x emissions by supplying a flue gas/air mixture through several passages. Flue gas is drawn from the combustion chamber by a blower.
ZINK et al., U.S. Pat. No. 4,004,875, discloses a low NO.sub.x burner, in which combusted fuel and air is cooled and recirculated back into the combustion zone. The recirculated combusted fuel and air is formed in a zone with a deficiency of air.
OPPENBERG et al., U.S. Pat. No. 4,575,332 discloses a burner having both oil and gas burner lances, in which NO.sub.x emissions are reduced by discontinuously mixing combustion air into the oil or gas flame to decelerate combustion and lower the temperature of the flame.
GRIFFIN, U.S. Pat. No. 2,918,117, discloses a heavy liquid fuel burner, which includes a venturi to draw products of combustion into the primary air to heat the incoming air stream to therefore completely vaporize the fuel.
In addition to MICHELSON et al., the other patents discussed above are also incorporated herein in their entireties, by reference thereto.